swift234

Two changes:
 - Fix a misunderstanding I had about ownership requirements in my previous patch:
   now any references to value-promoted self do a retain and use a ManagedValue,
   just like the semantic load path used to.  This is the change to visitLoadExpr
 - Second, change argument lowering to drop the "self" argument of normal class
   methods into a constant reference, instead of making a box for it.  This 
   greatly reduces the amount of SIL generated for class methods.

The argument lowering piece is somewhat hacky because initializations really want
to be dealing with memory, but it seemed like the best approach given the current
design.  Review appreciated.



Swift SVN r10984